Da rcgroups:
Q 5: I heard there are different types of head used on the Solo Pro. What are the differences?
A: The main change is a different type of links between swash plate and mixer arms on the blades. The older head uses "banana" shaped linkages, while the newer head uses "o-shaped" ones. Because of this change the small thin pins on the top of the head, that those links rest against, are placed on a different location on the plastic head. So the new type of links won't fit on an older type head, at least not very well. If you want to use the newer links, you would also need a newer type main shaft set. Since the older type of head is no longer current, when getting a spare main shaft set, it is very likely it will contain the newer type main shaft, which needs the "o-shaped" links. The pictures below show first the old type head, and then the new type.
In buona sostanza, ti devi procurare un nuovo albero con i pin al posto giusto per essere accoppiati ai nuovi link che hai comprato.
Consiglio di avere sempre a portata di mano la bellissima FAQ di rcgroups da cui è stato tratto il testo:
Nine Eagles Solo Pro/Bravo SX FAQ (Also valid for rebrands like Revell Proto Max etc) - RC Groups